So your galaxy S6 is 70.5 mm wide. People care about this for that is the width of your palm of your hand. We rarely care as much about the height of the phone. After width of the phone it is usually width > weight > height and thickness does not matter as long as it is reasonable which all modern phones are.
Here are 11 phones that are 70 mm or less in width and has a top end cpu. [Maybe not the latest 2019 cpu but 2017 or newer top end.] Links to GSM Phone Finder and a search result I already pulled up.
www.gsmarena.com
Notice the screen size are everything from 4.6" to 6.1"
Notice the Galaxy S8, S9 and S10e are on that list and are less wide than the S6.
S8, 68.1 mm wide and a 5.8" screen
S8, 68.7 mm wide and a 5.8" screen
S10e 69.9 mm wide and a 5.8" screen
Sony Xperia 5 68mm wide and a 6.1" screen
Google Pixel 4 68.8mm wide and a 5.7" screen
And if your wife wants something that feels significantly more handable for it is 1/2 a cm less wide the Sony Xperia XZ1 and XZ2 are only 65 mm wide. The downside is a 4.6" screen and 5.0" screen respectively and some people want a bigger screen and do not mind a half a cm wider phone. But for both men and women hands, half a cm is a big deal for our hands are only 7 to 8 cm wide as a "palm width" with the average women (and due to bell curves some women have smaller hands than the average male) and with men the average palm width is 8 to 9 cm wide (and due to bell curve some men have smaller hands than the average male.)
On the subject of a half a cm, remember her phone is 70.5 mm wide, well your pixel xl (aka the 1st gen) is 75.7 mm wide. Thus half a cm is the difference between unwieldly vs wieldly, and she may appreciate going down another half a cm to the Sony XPeria XZ1 size (65) , or maybe she is fine with the 68 to 70 range. She just needs to try things out for everyone is different and there is no shame with liking phones you can comfortably hold in your hand instead of feeling like you are stretching your hand so that you do not drop it.